Two endonucleases specific for pyrimidine dimers have been extensively purified from Micrococcus luteus. These enzymes with an isoelectric point of 4.7 and the other 8.5, act on the damaged strand generating a 3'-OH and 5'-P group 5' to the dimers. They are both specific for T less than or greater than T, C less than or greater than T and C less than or greater than C cyclobutane dimers. The first enzyme (IEP 8.5) acts in regions of the DNA molecule possessing hydrogen bond fragility and the second enzyme is specific for those pyrimidine dimers in more stable regions of the DNA molecule. Mutants lacking both or either of the two enzymes have been isolated and characterized with respect to the relationship between repair physiology and enzyme levels. BIBLIOGRAPHIC REFERENCES: Riazuddin, S. and Grossman, L. "Purification of Radiation Damage Specific Endonucleases from M. Luteus". J. Biol. Chem., in press (Aug./Sept.) 1977. Riazuddin, S. and Grossman, L. "Enzymatic Properties of Endonucleases Specific for Pyrimidine Dimers Containing DNA. J. Biol. Chem., in press (Aug./Sept.) 1977.